Creation of a Main-Belt Comet: Active Asteroid P/2021 L4

Active asteroid P/2021 L4 was discovered in June 2021 by Pan-STARRS in the outer asteroid belt [1], showcasing a pronounced dust tail and coma on images taken in June and July 2021, which were gone by September 2021. This activity occurred far from perihelion, but in 2024, recurrent activity has been confirmed upon L4’s approach to perihelion [2], confirming L4 to be a main-belt comet [3].

Archival observations of L4 following its discovery are available between 2021 and 2023 from various instruments, including the Very Large Telescope’s (VLT) FORS2 instrument, the Hubble Space Telescope’s (HST) WFC3, and the Canada-France-Hawaii Telescope’s (CFHT) MegaCam. Pre-discovery observations of L4 were found in images captured with the CFHT’s MegaCam in 2012 [4], but further search for pre-discovery data of L4 is hindered by the insufficient limiting magnitude of relevant surveys at the time. Serendipitously, one observation of L4 by Pan-STARRS during its 2014 perihelion passage could be identified, albeit at a poor signal-to-noise ratio.

Our analysis of these archival observations includes investigations into L4’s brightness, activity status, appearance, and tail properties across several epochs in order to constrain the causes and consequences of its 2021 activity. Aside from the 2021 and 2024 instances, no signs of activity are present in any other observations, and synchrone-syndyne analysis [5] indicates that the 2021 activity was short-lived, with an upper limit on the duration of no more than a few weeks.

Based on our findings, we believe that L4’s 2024 near-perihelion sublimation-based activity was enabled by the 2021 event creating the necessary conditions, and that L4 had not been active prior to its discovery in 2021. At main-belt distances, exposed water ice has a short survival time, which means that the existence of ice-sublimating main-belt comets requires such activation events [6], but this marks the first time that an activation event may have been captured observationally. Further insight will come from investigations into activation scenarios compatible with our 2021 observations of short-lived mass loss far from perihelion, such as impacts or rotational destabilisation.

 

References

[1] Wainscoat R., Cunningham C., Flewelling H., et al., Central Bureau Electronic Telegrams 2021, 4986, 1.

[2] Hsieh H.H., Sheppard S.S., & Thirouin A., Research Notes of the American Astronomical Society 2024, 8, 283.

[3] Jewitt D., & Hsieh H.H., The Asteroid-Comet Continuum, in: Comets III, Eds. K.J. Meech, M.R. Combi, et al., University of Arizona Press, 2024, 767-798.

[4] Ly K., Deen S., Nakano M., & Alexandersen M., Central Bureau Electronic Telegrams 2023, 5210, 1.

[5] Finson & Probstein, The Astrophysical Journal 1968, 154, 327-352

[6] Schörghofer N. & Hsieh H.H., Journal of Geophysical Research: Planets 2018, 123, 2322-2335.